1. Field of the Invention
The present invention relates to an ultrashort pulse laser oscillator, and more particularly to a tunable ultrashort pulse laser oscillator.
2. Description of the Related Art
It has heretofore known a method for oscillating ultrashort pulse laser in accordance with such a manner that a laser medium placed in a laser resonator is subjected to mode locking.
As an ultrashort pulse laser oscillator realizing such method for oscillating ultrashort pulse laser in accordance with a manner wherein a laser medium placed in a laser resonator is subjected to mode locking, it has been known an ultrashort pulse laser oscillator wherein, for example, a birefringent plate or the like is placed in a laser resonator containing a laser medium, and the birefringent plate or the like is mechanically rotated, whereby light output from the laser medium is reflected with respect to the laser medium to amplify the same thereby producing laser oscillation, so that ultrashort pulse laser is output from the laser resonator.
In a conventional ultrashort fs pulse laser oscillator as described above, when a mode-locked state which comes to be a trigger is once allowed to arise in the initial stage, oscillation of ultrashort pulse laser is maintained by means of so-called Kerr lens mode locking (self mode locking) wherein the mode-locked state continues thereafter by nonlinear optical effect of the laser medium placed in the laser resonator.
However, there have been such problems that when a conventional ultrashort pulse laser oscillator as described above is used, a birefringent plate or the like has to be mechanically rotated, so that a rotating mechanism for such rotation is required, and thus, the whole equipment must be unavoidably large-sized, as well as that there is a fear of no oscillation of ultrashort pulse laser in the case where a back-lash arises in the rotating mechanism.
Furthermore, in a conventional ultrashort pulse laser oscillator, when a room temperature where the ultrashort pulse laser oscillator has been placed changes by several degrees, whereby a length of a laser resonator changes by several microns in response to the temperature change of the several degrees, a mode-locked state is not maintained. Accordingly, there has been such a problem that ultrashort pulse laser cannot be stably oscillated by such a conventional ultrashort pulse laser oscillator as described above.
On one hand, it has been known a method for oscillating picosecond mode-locked laser being ultrashort pulse laser by responding correctly a frequency to be applied to a photoacoustic optical crystal to a length of a resonator in a conventional tunable laser oscillator such as a tunable laser oscillator wherein the photoacoustic optical crystal is used.
In a conventional tunable ps-laser oscillator as described above, however, a frequency to be applied to a photoacoustic optical crystal must be changed in response to wavelength tuning in case of laser oscillation, whereby it arises necessity for changing a length of the resonator in response to changes in a frequency to be applied to the photoacoustic optical crystal, so that a mechanically actuating section for changing a length of the resonator should have been constituted in the laser resonator.
In this respect, when a back-lash arises in the mechanically actuating section for changing a length of the resonator, there has been a fear of such a problem that no oscillation of picosecond mode-locked laser being ultrashort pulse laser is effected.
The present invention has been made in view of such problems involved in the prior art as described above, and an object of the invention is to provide an ultrashort pulse laser oscillator by which stable electrically tuned ultrashort pulse laser oscillation is realized without requiring provision of a mechanically actuating section such as a rotating mechanism for wavelength tuning.
Moreover, another object of the present invention is to provide an ultrashort pulse laser oscillator that is arranged to be capable of maintaining a mode-locked state, even if a length of a laser resonator changes by around several millimeters.
In order to achieve the above-described objects, an ultrashort pulse laser oscillator according to the present invention comprises a laser resonator composed of a mirror having a predetermined transmissivity on output side, and a total reflection mirror which does not transmit light, but reflect the same and disposed oppositely to the mirror; a continuous wave oscillation laser for inputting excitation laser beam into the laser resonator; a tunable laser medium capable of laser oscillation in a wavelength region over a predetermined range disposed in the laser resonator; a photoacoustic optical crystal having birefringent property as a crystal for selecting wavelength, which is disposed in the laser resonator and to which is input outgoing light from the tunable laser medium and which diffracts one of selected wavelength components and compose laser resonator for only selected wavelength; a piezoelectric element mounted on the photoacoustic optical crystal, which is distorted in response to a frequency of voltage upon application of the voltage and inputs an acoustic wave having a frequency in response to the distortion to the photoacoustic optical crystal; a power source for applying voltage to the piezoelectric element; and a control means for controlling a frequency of voltage to be applied to the piezoelectric element by the power source; wherein a frequency of voltage to be applied to the piezoelectric element by the power source is controlled by the control means, whereby the voltage having a frequency conforming to a distance defined between the mirror on outgoing side and the total reflection mirror is applied to the piezoelectric element by means of the power source, besides laser beam output from the mirror on outgoing side is utilized as outgoing laser beam from the laser resonator.
Therefore, when voltage having a frequency conforming to a distance defined between the mirror on output side and the total reflection mirror is applied to the piezoelectric element by means of the power source in accordance with the present invention, a similar phase-locked state to a mode-locked state arises, so that output laser beam from the laser resonator comes to be ultrashort pulse laser beam having a wavelength in response to the frequency of the power source conforming to a distance defined between the mirror on outgoing side and the total reflection mirror.
Furthermore, an ultrashort pulse laser oscillator according to the present invention comprises a laser resonator composed of a mirror having a predetermined transmissivity on output side, and a total reflection mirror which does not transmit light, but reflect the same and disposed oppositely to the mirror; a continuous wave oscillation laser for inputting excitation laser beam into the laser resonator; a tunable laser medium capable of laser oscillation in a wavelength region over a predetermined range disposed in the resonator; a photoacoustic optical crystal having birefringent property as a crystal for selecting wavelength, which is disposed in the laser resonator and to which is input outgoing light from the tunable laser medium; a piezoelectric element mounted on the photoacoustic optical crystal, which is distorted in response to a frequency of voltage upon application of the voltage and inputs an acoustic wave having a frequency in response to the distortion to the photoacoustic optical crystal; a power source for applying voltage to the piezoelectric element; and a control means for controlling a frequency of voltage to be applied to the piezoelectric element by the power source; wherein a frequency of voltage to be applied to the piezoelectric element by the power source is controlled by the control means, whereby the voltage having a frequency conforming to a distance defined between the mirror on outgoing side and the total reflection mirror is applied to the piezoelectric element by means of the power source, upon application of the voltage, the piezoelectric element is distorted in response to the frequency of the voltage applied, so that an acoustic wave having a frequency in response to the distortion is input to the photoacoustic optical crystal, and the photoacoustic optical crystal outputs outgoing light having a wavelength in response to the frequency of the power source conforming to a distance defined between the mirror on outgoing side and the total reflection mirror among the outgoing light from the tunable laser medium input as diffraction light diffracted in a predetermined direction, thereby outputting the diffraction light from the mirror on outgoing side as outgoing laser beam.
Therefore, when voltage having a frequency conforming to a distance defined between the mirror on outgoing side and the total reflection mirror is applied to the piezoelectric element by means of the power source in accordance with the present invention, a similar phase-locked state to a mode-locked state arises, so that output laser beam from the laser resonator, which is diffraction light obtained as a result of diffraction of the photoacoustic optical crystal in a predetermined direction comes to be ultrashort pulse laser beam having a wavelength in response to the frequency of the power source conforming to a distance defined between the mirror on output side and the total reflection mirror.
Moreover, an ultrashort pulse laser oscillator according to the present invention may comprise further an optical element, which is disposed in the laser resonator and compensates dispersion of diffraction light output from the photoacoustic optical crystal.
In the above-described ultrashort pulse laser oscillators according to the present invention, the above-described continuous wave oscillation laser may be a continuous wave oscillated solid laser, a continuous wave oscillated semiconductor laser, a continuous wave oscillation Ar ion laser, second harmonics of a continuous wave oscillation Nd solid laser or the like.
Furthermore, in the above-described ultrashort pulse laser oscillators according to the present invention, the above-described laser resonator may be constituted in a Z-holding type laser resonator, or an X-holding type laser resonator.
After starting short pulse generation, the wavelength can be tuned by changing radio frequency applying the AOTF. In which a radio frequency is not as same as frequency of the resonator. Therefore, fast and random frequency tuning is available by changing the radio frequency applying the AOTF with keeping the short pulse generation.
Moreover, in the above-described ultrashort pulse laser oscillators according to the present invention, the above-described tunable laser medium may be arranged in such that an end plane of incidence thereof has been Brewster cut, and the end plane of incidence is disposed at Brewster angle with respect to an optical path in the laser resonator.